A number of existing electronic fuel control systems use MAP and AAP as control input parameters. The MAP is a primary input in the derivation of manifold air density. The AAP has several uses in a fuel control system, including derivation of the blow back pressure at a cylinder exhaust port; derivation of the manifold vacuum pressure by a subtraction of AAP from MAP for use in control of ignition advance; and derivation of upstream pressure behind an internally constricted exhaust gas recirculation valve.
One known method of obtaining values for both MAP and AAP is to employ a MAP sensor for normal sensing of MAP, and to use the wide open throttle value of MAP as a reasonable approximation of AAP. However, this method has clear limitations, as it is rarely the case in modern highway travel that a vehicle is operated under wide open throttle conditions. Another known method of obtaining values for both MAP and AAP is to employ a respective sensor for each. This method is difficult to justify in terms of cost as the value of AAP need only be updated on an intermittent basis as changes in altitude or similar type environmental influences occur.
In addition, there is a further concern of maintaining calibration of a pressure sensor to assure that zero drift does not significantly affect the accuracy of the sensor output signal. A typical response to this concern has been to use a high-quality, relatively high cost sensor with bounded drift parameters. Such a sensor is calibrated once for its useful life at the time of manufacture. This response has the clear disadvantages of incurring increased cost with only limited assurance of sensor accuracy.
It would be desirable to include in a sensor the capability to perform regular or intermittent standardization to account for zero drift. In the instrumentation art the term "standardization" has the generally accepted meaning of a single-point, automatic re-calibration of those parts of the sensor subject to slow drift due to aging and effects of parameters other than the sensed parameter. In this context, "single point" refers to the zero drift phenomenon, and "automatic" can mean periodic, regularly scheduled, or other controlled sequence of calibration activity.
An objective, therefore, of the present invention is to provide the capability to sense both MAP and AAP in a unitary pressure sensor. A further objective is to include the capability to standardize the pressure sensor in its operation to compensate for drift.